Yellow is a special colour in Thailand. It is the birthday colour of the current King of Thailand (King Bhumibol Adulyadej (Rama IX), born 1927). King Bhumibol is well beloved and deeply respected by all Thais, old and young. Many people wear yellow clothes on Royal days of the year, to show this love and respect.
It is also traditional in Thailand to choose yellow when buying flowers for a Thai person who is/has been ill (especially people in hospital), to say "I hope you get well soon".
WARNING: Because Thai people believe many yellow flowers are
only for people who are ill (with HIV AIDS in particular), if a Thai person receives many yellow flowers on a non-Royal day (see King information above) they may feel embarrassed if they are with other Thai people when they receive them, because they may worry that the other Thai people may think .. “That person must
be ill". (But ... a few yellow flowers in a bouquet, basket or vase
arrangement with other colours is no problem – they can be very
beautiful).
For more detailed advice about choosing colours for a Thai, Thai-Chinese or Chinese person, see our Advice page